In fluid measuring apparatus where measurements of the characteristics of a fluid are made from a sample of a main stream thereof, a portion of the fluid present in a main conduit is usually diverted therefrom into a secondary conduit where the measurement is to be made. For example, in viscometer apparatus for measuring the viscosity of fluids using gravity fall techniques, as disclosed for example in U.S. Pat. No. 3,686,931, issued to Austin Norcross on Aug. 29, 1972, a portion of the fluid whose viscosity is to be measured is diverted into a second conduit where the viscosity measuring device is placed. The fluid is caused to come to rest within the secondary conduit and a weighted piston is permitted to fall freely under gravity through the fluid within the secondary conduit. The time of such fall over a known distance provides a measurement of the viscosity of the fluid.
Because the clearance between the free falling piston and the walls of the secondary conduit is normally relatively small, it has been found that impurities, such as dirt or other undesired materials suspended therein, often become lodged in such narrow clearance space and adversely affect the velocity of fall and, hence, the accuracy of the viscosity measurement. Moreover, the presence of impurities will adversely affect the operation of valves which are used in the apparatus. It is desirable, therefore, to provide a relatively clear fluid (i.e., a fluid free of such impurities) in which undesired particles, at least above a specified size, are removed from the fluid before it is diverted into the secondary conduit.
While such removal can be effected by placing a screen essentially perpendicular to the fluid flow into the secondary conduit, it is necessary periodically to clean the screen on which the undesired material has collected. In order to do this it is usually necessary to stop the operation of the viscometer and to disassemble the device in order to obtain access to the screen in order to clean and/or replace it. The device must then be reassembled before it can be placed back into operation. Such cleaning and/or replacement procedures can be sufficiently costly and time-consuming so as to make it desirable to provide for such removal in a more effective manner, without requiring the substantial disassembly of the device and the consequent down time which results.
Further, in such measuring systems, in order to assure that the falling piston has reached its desired initial position, a push rod is often utilized for moving the piston within the conduit to such position. The use of external push rods requires the use of a stuffing box which creates troublesome maintenance problems which it is desirable to avoid. On the other hand, if the piston is permitted to rise to its initial position in the conduit merely by utilizing the pressure introduced by the flow of the fluid into the conduit, it cannot always be assured that the piston has reached its correct initial position, since it is not possible in present-day apparatus of this type to determine the position of the piston within the conduit.
Moreover, in present-day devices the beginning of the free fall time period is normally determined by the closure of a valve, which prevents further entry of fluid into the conduit, and the end of the free fall time period is detected by a proximity circuit which, through appropriate magnets, causes the actuation of an electric circuit for providing a visual indication, such as a light, when the piston reaches its lowest point. Such a proximity detection device is disclosed, for example, in U.S. Pat. No. 3,677,070, issued to Austin Norcross, on July 18, 1972. Since the use of such proximity device adds to the cost of the apparatus, it is desirable to provide for an external indication of the piston position without the need for such a device and thereby reduce the cost while still maintaining adequate accuracy of the viscosity measurement. Such an external indication of the position of the falling piston permits an operator to know at all times the location thereof within the measuring conduit.
Further, in present day devices, when the flow is stopped so that the viscosity measurement can take place, the cessation of flow sometimes causes a sufficient change in fluid temperature to adversely affect the accuracy of the measurement. It is further desirable, therefore, to provide an apparatus in which such temperature changes are minimized.